1. Technical Field of the Invention
The present invention relates generally to the manufacture of earth boring bits, and more particularly to the manufacture of rotary bits through a molding process.
The present invention relates to a process utilized to manufacture rotary bits for drilling subterranean formations, as well as features created therein, by utilizing CNC machining to create one or more graphite parts used to cast the bit matrix with a single mold assembly. Such as assembly advantageously eliminates the need for multiple prefabricated components and artwork, such as performs and clay filling, as is known and used in the prior art fabrication process.
2. Description of Related Art
Fixed cutter drill bits known in the art include polycrystalline diamond compact (PDC) bits. The typical PDC bit includes a bit body which is made from powdered tungsten carbide infiltrated with a binder alloy within a suitable mold form. The particular materials used to form PDC bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. The PDC cutting elements used on these bits are typically formed from a cylindrical tungsten carbide substrate. A diamond table made from various forms of natural and/or synthetic diamond is affixed to the substrate. The substrate is then generally brazed or otherwise bonded to the formed bit body in a selected position on the surface of the body.
The materials used to form PDC bit bodies, in order to be resistant to wear, are very hard and are therefore difficult to machine. The shape and configuration of the bit bodies must accordingly, in most cases, be substantially defined during the bit body molding process. More specifically, the mold used in the molding process defines the size and shape of the gage of the bit body. The mold used in the molding process further defines the number and shape of the blades of the bit (along with the associated junkslots). Still further, the selected positions in the blade at which the PDC cutting elements are to be affixed to the bit body are also typically defined by the mold and formed substantially to their final shape during the bit body molding process.
Reference is now made to FIG. 1A which illustrates a junkslot plug mold 10 and to FIG. 1B which illustrates a junkslot plug 12, these mold structures being well known to those skilled in the art for use in producing a preform. When building a drill bit mold in accordance with known techniques of the prior art, the junkslot plug mold 10 and a junkslot plug 12 are machined in accordance with a desired design and specification. The junkslot plug 12 is placed within the junkslot plug mold 10 (turned upside down from what is shown in FIG. 1B) and the resulting assembly is then infiltrated with resin-coated sand that fills the open spaces 14 within mold 10 as defined by the shape of the plug 12. When the resin-coated sand cures, the junkslot plug can be removed from the mold.
The result of this preform molding process is shown in FIG. 2, where the material 16 as removed from the plug mold 10 is the cured resin-coated sand (which filled the open spaced 14 in the mold) and the material 18 is junkslot plug 12 which was inserted into the mold 10. The cured resin-coated sand material 16, after molding, becomes a set of sand junkslot formers 20. The material 16 for the sand junkslot formers 20 is carefully removed from the junkslot plug 12 material 18 for subsequent use in the actual bit mold from which the bit is cast. This will be described in more detail below. What is important to consider is the time, effort and expense which is expended in connection with defining and producing the sand junkslot formers. There would be an advantage if the perform molding process for producing the sand junkslot formers could be eliminated.
Reference is now made to FIG. 3 which shows how the sand junkslot formers 20 have been inserted into a bit mold 26 from which an actual bit will be created. The bit mold 26 includes a bottom portion 30 into which a number of cutter pockets 32 have been formed and an upper portion 36 (having a ring shape) defining the gage of the bit. Each cutter pocket includes a seat 37 and a face 39. The bottom and upper portions 30 and 36 of the bit mold 26 are assembled together as shown, and the sand junkslot formers 20 are then securely inserted into the assembled mold 26 in proper alignment with the cutter pocket 32 locations. More specifically, the sand junkslot formers 20 are positioned between sets of adjacent cutter pockets 32 to define the location of the junkslots for the bit and thus further define the shape of the bit blades associated with each set of cutter pockets.
Although described above in the context of forming junkslots, it is well known in the art that other preform pieces can be cast in the manner described using resin-coated sand. Such preforms are employed to define, in addition to junkslots, the internal fluid passages to deliver drilling fluid to the bit face, as well as cutter pockets, cutter faces, and nozzle displacements.
Reference is now made to FIG. 4. At this point in the bit manufacturing process, a cutter shaping element called a “displacement” 40 is installed in the bit mold at each of the formed cutter mounting positions (i.e., the cutter pockets 32). The displacement 40 is a cylindrical graphite piece which represents a PDC cutter. This displacement is placed resting in the seat 37 at each of the cutter pocket 32 location and is secured in a position such that a first end 41 of the displacement rests against the face 39 of the pocket facing towards the sand junkslot former 20. Each displacement 40 has the same size and shape as the polycrystalline diamond compact cutter which has been designated for use at that pocket 32 in the to-be-molded bit. Thus, the displacement 40 is used to form the shape of the PDC cutter mounting positions during the bit body molding process.
To extent there are any imperfections in the bit mold, for example due to problems with the size, shape and/or configuration of the sand junkslot formers 20, or for example due to problems with the relationship between the installed sand junkslot formers 20, cutter pockets 32 and installed displacements 40 (for example, at the seat 37 or face 39), these imperfections must be addressed prior to molding. It is common in the art to use a clay material 44 to fill any noted voids, misalignments, imperfections, and the like, in the bit mold. For example, clay 44 can be used to fill voids between the front first end 41 of the installed displacement 40 and the installed sand junkslot former 20 (generally at the seat 37 or face 39 locations). Clay 44 can also be used to fill the space between the installed sand junkslot formers 20 and the bottom and upper portions (30 and 36, respectively) of the mold. Imperfections, undercuts, edges, and the like may also be addressed through the selective application of filling clay 44.
The molded bit includes a bit body formed using a matrix of hard particulate material, such as tungsten carbide, that is infiltrated with a binder, generally copper alloy or similar material. The bit body is cast around a cylindrical piece of steel, also known as “blank,” which is used for internal reinforcement of the bit body matrix. The blank, along with the sand pieces and graphite cutter displacement cores, are placed in the mold in order to cast the bit. This assembly of components is then filled with tungsten carbide powder that is infiltrated with binder in a furnace. During cooling, the matrix bonds to the blank. Once the assembly has cooled, the graphite of the mold 26 is chipped away and all of the sand preforms (such as junkslot formers 20), clay 44 artwork, and graphite cutter cores (displacements 40) are removed and cleaned away leaving the bit body. A threaded pin connection, also termed an “upper section”, is then welded to the blank of the bit body. The upper section is used to attach the bit to the drive apparatus, normally a drill collar or a downhole motor. The PDC cutting elements are then bonded to the bit face, in the openings left by the removed displacements, by brazing. The process for casting the bit as described in this paragraph is well known to those skilled in the art.
The building process to fabricate a matrix drill bit is very costly and quite complex. This process requires the fabrication of a mold that is then used to cast the bit. The blank and sand pieces are individually designed and fabricated, and the design and configuration of these components are often times revised thus requiring costly production time in lieu of process adjustments that are needed to introduce new and different preforms. For many years, bit molds have been machined to a standard bit profile. Sand preforms cast from the junkslot plug are then glued between each blade location in the mold in reverse, along with all other graphite plugs and sand performs, by skilled technicians employing various files and sculpting tools. These technicians also employ the use of a special bit molding clay comprised of graphite powder, bee's wax, and permaplast modeling clay. This clay is used to correct any imperfections in the mold. There is a need in the art for a simpler, less expensive, and more accurate process for bit mold creation.
Reference is also made to U.S. Pat. Nos. 5,358,026, 6,073,518, and 7,159,487, the disclosures of which are hereby incorporated by reference herein.